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HEPA Furnace Filter for Cleanroom AHU: Selection and Sizing Guide
If you’re integrating a HEPA furnace filter into a cleanroom AHU, your real challenge is sizing and specifying it so you hit ISO 14644 targets without oversizing fans or burning energy. The short answer: treat the AHU HEPA stage as part of a prefilter–HEPA chain, size by required airflow and face velocity, and design around pressure drop and access for testing and change-out. This guide gives HVAC engineers and cleanroom designers a practical framework.
HEPA Filter Integration in Cleanroom AHU Design
Centralized vs Distributed HEPA Filtration Approaches
You can use a hepa furnace filter in two main strategies:
Centralized HEPA in the AHU
- One HEPA bank filters supply air for multiple rooms.
- Easier to maintain in a single location, but any failure can affect many spaces.
Distributed terminal HEPA
- HEPA at room ceilings (FFUs or terminal modules).
- Better zoning, easier to tailor by cleanliness class.
Many modern designs use a hybrid: a centralized hepa air filter stage in the AHU for bulk cleaning, plus terminal HEPAs in higher-grade rooms for final polishing.
AHU-Mounted HEPA vs Terminal HEPA Configurations
Trade-offs:
- AHU-mounted HEPA
- Pros: single test point, fewer filters, simpler duct routing.
- Cons: more duct surface between HEPA and room; risk of downstream contamination; harder to meet strict Grade A/B at the diffuser.
- Terminal HEPA
- Pros: filtration right at room boundary, ideal for ISO 5/Grade A/B.
- Cons: higher filter count, more complex ceiling work.
In our experience, using a hepa furnace filter (or HEPA bank) in the AHU works well up to ISO 7–8 and as a pre-stage for ISO 5–6 with terminal filters on top.
Pre-Filter Strategy to Extend HEPA Lifespan
To protect the AHU hepa filter filters, use at least two prefilter stages:
- Rough filter (e.g., ISO coarse 60% / MERV 7–8) at AHU intake.
- Fine filter (e.g., ePM1 70–80% / MERV 13–14) before HEPA.
Advantages:
- Extends HEPA life from 2–3 years to potentially 5–7 years in clean environments.
- Keeps pressure drop stable, reducing fan energy and noise.
From the projects we’ve completed, facilities that upgraded to a two- or three-stage prefilter strategy cut HEPA replacement rates by 30–40% with minimal added prefilter cost.
Sizing HEPA Filters for AHU Applications
Airflow Calculation Methods per Cleanroom Volume
Start by defining required supply airflow:
- For ISO-class cleanrooms:
- Use air change rate: e.g., 20–40 ACH for ISO 7–8, 40–240 ACH for ISO 5–6 (depending on process and Annex 1 guidance).
- Formula:
- Supply CFM = Room volume (ft³) × ACH / 60
- Or m³/h = Room volume (m³) × ACH
Sum all room loads handled by the AHU to establish total CFM/m³/h that your hepa furnace filter bank must handle.
Face Velocity Recommendations (250–500 FPM)
HEPA face velocity influences:
- Pressure drop.
- Efficiency at MPPS.
- Noise and turbulence.
Typical ranges:
- Central AHU HEPA banks: 250–400 FPM(1.3–2.0 m/s).
- Terminal HEPAs: often lower, ~90 FPM (0.45 m/s) for unidirectional flow.
In our experience, designing AHU HEPAs around 300 FPM (1.5 m/s) is a good compromise between size and ΔP for most industrial cleanrooms.
Standard Filter Sizes and Custom Sizing Options
Common HEPA module sizes:
- 24″×24″×11.5″ (610×610×292 mm).
- 24″×48″×11.5″ (610×1,220×292 mm).
Use:
- Standard sizes whenever possible to avoid long lead times and high custom pricing.
- Custom modules only where AHU footprint or duct geometry demands it.
Remember that each filter hepa filter module has a nominal CFM rating; ensure the sum of module capacities exceeds AHU design airflow at your selected face velocity.
Pressure Drop Considerations for Energy Efficiency
Initial vs Final Pressure Drop Specifications
For each hepa furnace filter, consider:
- Initial ΔP: often 1–1.5 in. w.g.(250–375 Pa) at rated velocity.
- Final (end-of-life) ΔP: typically 0–2.5 in. w.g.(500–625 Pa), depending on design.
You should:
- Size fans and VFDs for final ΔP plus duct and coil losses.
- Use ΔP sensors across the HEPA bank for monitoring and change-out planning.
Impact on Fan Motor Sizing and VFD Requirements
Fan energy is directly tied to total static pressure:
- Motor sizing must handle worst-case (final filter plus safety margin).
- VFDs allow you to ramp fan speed as filters load, keeping airflow constant.
From the projects we’ve completed, ignoring HEPA ΔP in fan selection is a common mistake—often discovered only when commissioning reveals low CFM at high fan speed.
Balancing Filter Life Against Energy Consumption
You can extend HEPA life by allowing higher final ΔP, but:
- Higher ΔP = more fan power (kW) and operating cost.
- There is a trade-off between saving filters and spending more on electricity.
Best practice:
- Run cost analysis over 5–10 years to find optimal change-out ΔP where combined filter and energy cost is minimized, not just filter cost alone.
Frame and Housing Options for AHU Installation
Front-Loading vs Side-Access Housing Designs
Types of HEPA housings for AHUs:
- Front-loading:filters loaded from the air-entering side; common in plenums.
- Side-access:filters slide in from a side service door; useful for tight AHU rooms.
Selection depends on:
- Access space in mechanical rooms.
- Whether filters will be changed from an equipment corridor or within the AHU itself.
Stainless Steel vs Powder-Coated Aluminum Frames
Filter frames and housings:
- Stainless steel (304/316):
- High corrosion resistance; preferred for pharma, food, corrosive or wash-down environments.
- Heavier and more expensive.
- Powder-coated aluminum or galvanized steel:
- Lighter, lower cost.
- Suitable for many industrial cleanrooms with mild conditions.
In our experience, stainless housings with standard hepa filter frames offer a good balance in pharma and high-humidity applications.
Sealing Methods: Fluid Seal, Gasket, Clamping
Sealing is as critical as filter media:
- Fluid (gel) seal:knife-edge on housing mates into gel-filled channel on the filter; excellent seal, common in GMP installations.
- Gasket seal:neoprene/EPDM gasket compressed by clamps; simpler, common in industrial systems.
Regardless of method:
- Design for uniform compression around each hepa air filter
- Include clamping and inspection points to verify no visible gaps.
Special Considerations by Cleanroom Type
High-Temperature HEPA for Heat-Treat Environments
For processes with high exhaust or recirculation temperatures:
- Use high-temp HEPA furnance filters rated up to 250–400 °C (glass fiber media, stainless steel frames).
- Install them in AHUs or exhaust trains where air is cooled below rating, or use dedicated high-temp stacks.
Verify:
- Temperature profiles throughout the system.
- Compliance with relevant fire and mechanical codes (e.g., IBC, NFPA).
Chemical-Resistant HEPA for Corrosive Atmospheres
For corrosive gases (e.g., acid/alkali fumes in electronics, plating, or chemical processes):
- Specify corrosion-resistant housings and frames.
- Consider PTFE-coated frames or special gaskets.
- Use upstream scrubbing/neutralization where needed; HEPA filters target particles, not gases.
In our experience, corrosion on frames and housings is a leading cause of premature filter hepa filter replacement in chemical-heavy industries.
Antimicrobial-Treated HEPA for Biotech Facilities
In biotech or high-bio-load areas:
- Some vendors offer antimicrobial-treated HEPA media or coatings.
- These are a supplement, not a replacement, for proper maintenance and cleaning.
Assess:
- Regulatory acceptance (some drug products may restrict biocidal treatments in air paths).
- Added value vs cost and complexity.
Installation and Commissioning Checklist
Ductwork Transition and Diffuser Requirements
To ensure uniform distribution downstream of a hepa furnace filter bank:
- Provide straight duct sections (5–8 duct diameters) between HEPA and main branches, where possible.
- Use turning vanes and diffusers to avoid dead zones and turbulence at room diffusers.
Poor transitions can cause uneven velocities at terminal points, even if total CFM is correct.
Access Door Positioning for Maintenance
Plan maintenance access early:
- Install access doors upstream and downstream of the HEPA bank.
- Ensure technicians can reach all filters without unsafe climbing or disassembly.
From the projects we’ve completed, undersized or misplaced access doors are a major maintenance pain point—often discovered after commissioning.
Pressure Monitoring Port Installation
For each HEPA bank:
- Install pressure taps upstream and downstream.
- Route to differential pressure gauges or transmitters.
This allows:
- Continuous HEPA performance monitoring.
- Data-driven change-out decisions instead of arbitrary calendar intervals.
Frequently Asked Questions
Can HEPA filters be installed in existing non-cleanroom AHUs?
Yes, but with caveats:
- The AHU and ductwork must be cleanable and tight enough to justify HEPA-level air.
- Fans must have enough static pressure capacity for HEPA ΔP.
- Proper housings and seals must be added; you can’t just drop a hepa furnace filterinto a residential-style frame.
Often, upgrading to HEPA in an existing AHU is feasible for ISO 8–7 support areas, but for higher classes, you may still need terminal HEPA units.
What pre-filter efficiency extends HEPA life most effectively?
A good combination:
- Coarse prefilter (MERV 7–8 / ISO coarse) to catch large dust.
- Fine filter (MERV 13–14 / ePM1 60–80%) before HEPA.
This setup typically offers the best balance between prefilter cost and extended HEPA lifetime. Some pharma projects also add an intermediate stage (e.g., MERV 11) for very dusty environments.
How do I calculate the required number of HEPA filters for my AHU?
Steps:
- Determine total supply airflow required (CFM or m³/h).
- Select target face velocity (e.g., 300 FPM / 1.5 m/s).
- Calculate total filter face area needed: Area = Airflow / Velocity.
- Divide by the face area of one module (e.g., 4 ft² for 24×24) to get module count.
Always add a safety factor (10–20%) to allow for future airflow increases and to keep velocity within optimal efficiency/ΔP ranges.
A well-specified hepa furnace filter stage turns your AHU into a reliable backbone for cleanroom air quality. By sizing filters on airflow and face velocity, designing around pressure drop and housing details, and tailoring media types to your thermal and chemical environment, your hepa filter integration will support both ISO-class performance and long-term operating cost targets.
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We'll call you
In order to serve you better, if you could kindly send an e-mail to nick@biogreenness.com for questions and details.
